1. Field of the Invention
The present invention generally relates to an optical scanning device and an image forming apparatus including the optical scanning device.
2. Description of the Related Art
To achieve high speed printing and high image quality with a laser color image forming apparatus, it is generally necessary to accurately rotate a polygon scanner at a speed as high as 25000 rpm or more. Meanwhile, to reduce the diameter of a laser beam and thereby improve image quality, it is necessary to increase the inradius or the length in the main scanning direction of a polygon mirror of a polygon scanner. For these reasons, the workload of a polygon scanner in a laser image forming apparatus is becoming higher and higher.
This high workload in turn increases the power consumption of a polygon scanner and the resultant heat adversely affects optical elements such as a scanning lens. For example, the heat increases the temperature of a scanning lens located closest to the polygon scanner. The heat from the polygon scanner is transferred by conduction via an optical housing or by radiation to the scanning lens. The scanning lens is not uniformly heated. Instead, the heated scanning lens tends to have a temperature distribution particularly in the main-scanning direction or the length direction because of the difference in distance from the heat source (polygon scanner), the difference in thermal expansion coefficient of the materials, or air current.
The temperature distribution in the main-scanning direction affects the shape accuracy and refractive index of the scanning lens, thereby changes the position of a laser beam spot, and therefore reduces image quality. This problem is particularly prominent in a scanning lens made of plastic with a high thermal expansion coefficient.
In a laser color image forming apparatus, multiple laser beams corresponding to respective colors (yellow, magenta, cyan, and black) are used. Therefore, in addition to the temperature distribution of a scanning lens, the temperature difference between optical scanning systems corresponding to the respective colors may also cause a problem. The temperature difference affects relative positions of laser beam spots corresponding to respective colors and thereby causes a color shift of an image.
Also, temperature rise caused by a highly-loaded polygon mirror induces minute movement of components of a rotating body (particularly, the polygon mirror that has a high mass ratio), changes the balance of the rotating body, and thereby causes vibration. If the thermal expansion coefficients of components (e.g., a polygon mirror, a flange on which a rotor magnet is fixed, and a shaft) of a rotating body are different or if the tolerance and fixing method of the components are not properly managed and examined, minute movement of the components (a change in balance of the rotating body) occurs during high-temperature, high-speed rotation and as a result, the vibration is increased. Further, the vibration is amplified and transmitted to an optical element (e.g., a reflective mirror) in the optical scanning device and causes banding, image degradation, and noise.
To solve the above problems, use of an oscillating mirror, which employs resonance phenomena, instead of a polygon mirror deflector has been proposed (see, for example, patent documents 1 through 4). An oscillating mirror consumes less power, and therefore using an oscillating mirror reduces temperature rise of a scanning lens used in an optical scanning device. This in turn reduces vibration of an optical scanning device and reduces temperature difference between optical scanning systems in a laser color image forming apparatus.
However, because an oscillating mirror uses the resonance phenomenon of a torsion beam to obtain a practical oscillation amplitude or angle, the size of a movable mirror of an oscillating mirror is very small and is about 1/10 to ⅕ of a related art polygon mirror in area. This small size of a movable mirror makes it difficult to reduce the diameter of a laser beam spot.
Also, it is difficult to accurately adjust the position of a movable mirror with respect to a reference surface of an optical housing on which a scanning imaging lens is mounted. In a related art polygon mirror, because a machined part and a bearing are formed as a single unit, the machining accuracy determines the accuracy of the polygon mirror. Meanwhile, a movable mirror of an oscillating mirror is generally produced by a semiconductor process and is mounted on a separately-produced bracket. The positional accuracy of a movable mirror tends to be reduced when it is mounted on the bracket.
With a related art optical scanning device using such an oscillating mirror, it is difficult to achieve optical characteristics corresponding to those of a polygon mirror. For example, it is difficult to achieve a desired laser beam diameter on a target surface which is small enough (less than 80 μm in the main- and sub-scanning directions: 1/e2 of peak light intensity) to form a high-quality image with a resolution of 600 dpi or higher. This is caused by the small area and low positional accuracy of an oscillating mirror. That is, with such an oscillating mirror, a laser beam from a light source may be eclipsed on the movable mirror and the amplitude center of the movable mirror and the center of a scanning lens may become misaligned. Thus, with a related art oscillating mirror, it is difficult to achieve accuracy required in optical design.    [Patent document 1] Japanese Patent Application Publication No. 2005-202321    [Patent document 2] Japanese Patent Application Publication No. 2007-058205    [Patent document 3] Japanese Patent Application Publication No. 2007-171854    [Patent document 4] Japanese Patent Application Publication No. 2007-233235
In a typical color image forming apparatus, latent images corresponding to black, yellow, magenta, and cyan components of a color image are formed on corresponding photoconductive drums, the latent images are visualized with toners of corresponding colors to form toner images, and the toner images are superposed on a recording medium such as paper and fused onto the recording medium to form the color image (see, for example, patent document 5). In these years, such an image forming apparatus has come to be popularly used as an on-demand printing system for low-cost (or small-scale) printing. Accordingly, there is a growing demand for an image forming apparatus with improved productivity and improved image quality.    [Patent document 5] Japanese Patent Application Publication No. 2004-286852
In an image forming apparatus as described above, a laser beam emitted from a light source is caused to fall on a deflection surface of a deflector by an optical system including multiple optical elements and the laser beam deflected by the deflector is focused on a target surface by a scanning optical system including multiple optical elements such as scanning lenses. To form a high-resolution image with no defect such as a color shift using the image forming apparatus, it is necessary to accurately adjust the optical positional relationships between optical elements including the light source and the deflector and the target surface and thereby to cause the laser beam to accurately fall on the target surface. Also, because optical characteristics of an image forming apparatus are affected by manufacturing errors, mounting errors, and aging deterioration of components including optical elements, it is necessary to make adjustments unique to the image forming apparatus when assembling the image forming apparatus or during regular checkups to maintain the optical characteristics.